Make private field name parsing ecma 262 compliant

[amcasey]

This file has parse errors, but probably shouldn't.

Considerations:

• Improve escape sequence handling in private names #50856 added support for starting with a unicode escape
• The names #ZW__NJ and #ZW__J contain \u200c and \u200d, respectively (not here, but in the file)
• Support for extended unicode escapes (i.e. \u{6F}, rather than \u006F) depends on ES2015, but we appear to parse the file as ES5.
• Converting the extended unicode escapes to regular unicode escapes does not seem to eliminate the errors

Discovered as part of #50835

[DanielRosenwasser]

The extended Unicode escapes in ES5 issue is intermingled with regular identifiers; the current direction I had for extended escapes was to conditionally add a property and mark such nodes as ContainsES2015, but there were some issues in the transformer that I ran into.

If someone can beat me there, great! Though give me a heads up if you do decide to start working on it.